Spatial lag

[mehak-sachdeva]

• [x] All declared geometries are geometry(Geometry, 4326) for general geoms, or geometry(Point, 4326)
• [x] Docs for public-facing functions are written
• [x] New functions follow the naming conventions: CDB_NameOfFunction. Where internal functions begin with an underscore
• ~[ ] Video explaining the analysis and showing examples~ -> scope of these requirements has changed, no longer required ~- [ ] Analysis Documentation written template~ ~- [ ] Hand-off document for camshaft node written~
• [x] If function is in Python, code conforms to PEP8 Style Guide

closes https://github.com/CartoDB/crankshaft/issues/173

[mehak-sachdeva]

Reference: https://github.com/CartoDB/crankshaft/issues/173 This function will be a supplement to machine learning functions mostly so not sure if the camshaft nodes will be handled separately or not. @andy-esch : could you please review?

@stuartlynn : for visibility